Macrocyclic polyester oligomers (MPOs) have unique physical properties that facilitate the manufacture of polyester products. MPOs demonstrate certain processing advantages of thermosets, yet can be polymerized to form thermoplastic polyesters which provide superior toughness, excellent chemical resistance, high heat resistance, and are thermoformable, paintable, bondable, weldable, and recyclable. For example, MPO resins are available as easy-to-handle solid pellets that melt into a low viscosity fluid when heated. The low melt viscosity allows the MPO resin to easily fill molds or permeate fabrics to make prepregs. Furthermore, certain MPOs melt and polymerize at temperatures well below the melting point of the resulting polymer. Upon melting and in the presence of an appropriate catalyst, polymerization and crystallization can occur virtually isothermally, without significant heat generation and without production of volatile organic compounds (VOCs) or other harmful emissions. The polymerized product can be released without cooling the mold, and the time and expense required to thermally cycle a tool is favorably reduced.
Laboratory experiments demonstrate it is possible to perform a (reversible) ring-opening polymerization of MPO in the presence of an appropriate catalyst. However, the development of industrial processes and equipment for the catalytic polymerization of MPO in the manufacture of polymer parts and composites has been limited. The design of industrial polymerization processes is difficult, for example, because the polymerization rate and maximum conversion often vary as functions of the geometry of the part, the reaction temperature, the reaction time, the method of mixing during reaction, the type and concentration of catalyst, and/or the concentrations of other components. There are also material handling and heating concerns that must be considered in the design of industrial manufacturing processes. Even where it is possible to accurately model the heat and mass transfer and reaction kinetics for a given catalytic system, it may be impossible to design a process that is versatile enough to provide adequate polymerization rates and conversions for the successful manufacture of certain products.
Thus, there is a need for catalytic systems of increased versatility to better control the onset and speed of polymerization of MPOs. For example, there is a need for catalytic systems which do not appreciably begin to catalyze MPO polymerization until an appropriate time, and which allow the polymerization to take place quickly, completely, and relatively homogeneously throughout the reaction mixture once reaction begins. There is also a need for simpler, more versatile, and lower-cost processes for manufacturing thermoplastic parts.